1. Field of the Invention
The present invention relates generally to a control system for an internal combustion engine, and more particularly to a control system for a fuel injected, natural gas engine designed to protect the engine, allow the engine to be started, control the engine speed, and maintain desired exhaust emission levels.
2. Description of the Related Art
Various methods for electronically controlling the operation of internal combustion engines are known in the art. For example, many automotive engines utilize electronic control systems to control various engine operating parameters, such as fuel flow rate and air/fuel ratio. The control system will often utilize sensors to monitor engine operating conditions and will output electronic signals to control the operation of the engine according to a control process.
A typical control system may include an exhaust gas oxygen sensor, for example, to provide feedback to the control system for adapting the engine's air/fuel ratio to a desired value, such as the stoichiometric air/fuel ratio. At the stoichiometric air/fuel ratio, all the air in the combustion chambers completely oxidizes all of the injected fuel during combustion, leaving no oxygen in the exhaust gasses. The amount of oxygen in the exhaust, therefore, can provide an indication as to the relationship between the actual air/fuel ratio supplied to the engine and a desired air/fuel ratio such as the stoichiometric air/fuel ratio.
Electronic control systems have also included fuel control systems to meter the amount of fuel delivered to the engine over a range of operating conditions. Typically, a fuel control system, or "governor," will generate a fuel command signal indicative of the rate at which fuel is to be delivered to the engine to maintain the actual engine speed at a desired engine speed. The actual and desired engine speeds can be monitored by sensors, such as an engine speed sensor and a throttle sensor.
Prior engine control systems have encountered problems in controlling engine operating parameters, however, because they have often included an independent fuel and air/fuel ratio controller. In this type of control system, the fuel flow is set based on desired engine speed and the air/fuel ratio is set independently according to a predetermined value. Such control logic fails to protect the engine from receiving an unacceptably rich air/fuel mixture when there is insufficient air available at the intake, since the fuel level is set independently of the air/fuel ratio. This can occur, for example, when a step load is applied to the engine.
Furthermore, natural gas engines have encountered problems in operation when the natural gas supplied to the engine is not of uniform molecular weight. Because the molecular weight of natural gas is correlated to its energy content, the operation of a natural gas engine will vary considerably depending on the molecular weight of the natural gas being used. In engines which do not account for fuel molecular weight, when "hot" fuel, which has a high energy content and high molecular weight, is injected into the engine, the engine immediately receives a rich air/fuel mixture because the fuel gas has a high energy to volume ratio and the intake manifold pressure remains constant.
A rich air/fuel mixture can increase the possibility of detonation, or engine knock, especially with a high engine load, which can cause serious mechanical failures. Also, a rich air/fuel ratio can result in high emissions of nitrogen oxides. At a minimum, therefore, the engine's exhaust emissions will vary considerably, possibly causing the engine to be out of compliance with government emission regulations.
Independent governor and air/fuel ratio controllers also require additional logic for starting and do not protect the engine from overload. Prior engines, for example, have controlled fuel pressure during starting using a simple ramp function which can cause flooding unless additional control logic is provided to adapt the ramp according to ambient conditions such as air temperature and fuel temperature.
Prior engines have also suffered from defects relating to a single controller of pilot and main fuel flow rates. Engines with large combustion chambers, for example those which burn natural gas, often include a precombustion chamber in which a small quantity of gas is ignited and subsequently directed into a main combustion chamber. The ignited gas covers a much larger volume than a spark from a single spark plug, and can therefore ignite the fuel in the main combustion chamber much more rapidly. If the air/fuel ratio in the precombustion chamber is near the stoichiometric air/fuel ratio, ignition of the fuel occurs most rapidly. However, many engines utilize a single fuel flow controller to inject fuel into the main combustion chamber and the precombustion chamber. Thus, when the air/fuel ratio in the main combustion chamber is rich, the air/fuel ratio in the precombustion chamber will also be rich. Consequently, rapid burning is inhibited because the air/fuel ratio in the precombustion chamber is considerably in excess of the stoichiometric air/fuel ratio. Further, because excess pilot fuel is delivered to the precombustion chamber, fuel consumption has been greater than necessary.
U.S. patent application Ser. No. 08/312,919 entitled "Swirl Flow Precombustion Chamber" and filed concurrently herewith discloses a combustion system including a precombustion chamber that may be used with the present invention. The subject matter of the copending application is incorporated herein by reference.